Method of making zinc sulphide



July 27 ,1926. 1,594,001

Fy G. BREYER` ET A1. MTHOD OF' MAKING ZINC SULPHIDE vF116@ April 7. 1925s sheets-sheet 1 ATTORNEYS July 27 1926. Y f 1,594,001

F.-G.BREYER E-r Al.

METHOD OF MAKING ZINC SULPHIDE Filed April 7. 1925' 3 sheets-sheet 2 l fINVENToRs nM/c G-.M/

. ATTORNEYS July 27 1926. 1,594,001

v F. G. BREYER ET AL METHOD OFMAKING ZINC SULPHIDE Filed'April v, 1925 ssheets-sheet s 'Patented July 27, 1926.

f "UNITED srrATEs #PATENT oFFylCEQ EEANxG. BEEYEE, 0E PALMERTON,

COMPANY, OE'NEWYORK,

N. Y.,-A CORPORATION OF NEW JERSEY.

l METHOD 0E lJizrAxINer zINc sULrHrDE. i

' Appiication nieaAprii 7, 1925.v serial No. 21,276.

' This invention relates to zinc sulphide pigment and has for its objectthe provision of an improved method of making zinc sulphide in the formof pigment.

When a mixture of -Zinc oxide pigment and sulphur is heated undernon-oxidizing conditions, as for example in a closed. vessel, to atemperature 'in excess of about 400 C., a reactionreadily takes placebe'- tween the zinc oxide and sulphur resulting Jin the formation ofzinc sulphide of appropriate physical characteristics for use as apigment. The chemical reaction may be represented by the followingequation:

The reaction between the zinc oxidepig ment and sulphur begins at atemperature slightly above 100 C. The reaction velocity increases withrise in vtemperature becoming extremely active atl temperatures of G-400C., although not suticiently rapid to eii'ect complete conversion of thezinc oxide to zinc sulphide in a mixture containing approximately thecalculated theoretical proportion of zinc oxide and sulphur. to loss ofsulphur from the charge, the Vapor tension of ,sulphur being high atthese temperatures so lthat considerable sul- ,phur vapor is carriedaway with the sulphur dioxide gas resultingl from the reaction. Thisdeficiency of sulphur in the charge as a result of volatilization may beovercome by using an excess of sulphur in the mixture. `Practicallycomplete converl ,sion of the zinc oxide pigment to zinc sulment ofrelatively small particle size.

(phide pigment can be accomplished at 'a temperature of 450 C. with amixture containing 25% more than the calculated proportion of sulphurand a zinc oxide pig Increase of the temperature to 600o C. and highercorrespondingly increases the reac tion velocity and gives moreconsistently high conversion of zinc oxide to zinc sulphide. Y

The neness of particle subdivision ot the zinc oxide pigment exercises'a marked influencel on the rapidity with which the reaction' proceedsand its degree of com'- pleteness. Thus, withlthe usual grades of vThisappears to be yduevin part- American or French processes zinc oxide,

l AND cLA'Y'roN'W.- BARBER, or nowisiuiNsTowN, PENNSYLVANIA, AssIeNns To'THE NEW JEEsEY zrNc whose average particle size is approximately0.4-0.5 micron, the reaction'proceeds relatively much slower than withzinc oxide of an average particlesize of 0.25 micron, or less, such asdescribed yin the United States 'patentsof Breyer, Gaskill `andSingmaster No. 1,522,098, patented January 6th, 1925. Withreactiontemperatures of from (300C-800o C., rapid and efficientconversion of the zinc oxide pigment to Zinc sulphide pigment is eectedwhen using Zinc oxide pigment of an average particle size of 0.25 micronor less. A

If a zinc sulphide pigment product is desired having the maximumpossible whitenes's'and brightness, it is essential that only extremelypure zinc oxide and sulphur be used. The most satisfactory pigment inthisrespect is that made `from high grade slab zinc. Similarly,resublimed flowers of sulphur should be used. The zinc sulphidepigment-made from less pure zinc oxide and sulphur is usually decient incolor and brightness. If vthese properties of color and brightnessare'not of the first importance, lower grade 'raw materialsmay be used.i

@The zinc sulphidevobtained by heating a provement in the color of thezinc sul` phide pigment can be brought about by subjecting the raw zincsulphide,y produced ,as hereinbefore described, to a reheating lorremufllingtreatment.

of the change produced by this reheating or remui'lng treatment has notbeen determined. No signilicant change in the chemical composition ofthe zinc sulphide pigment takes place, nor is there any apparent changein physical properties, than the marked improvement in color.

zinc oxide The exact nature Y other The reheating ior remulingtreatment,

should be carried out under non-oxidizing cond1t1ons, for example, 1nanv atmosphere l, of superheated steamer other non-oxidizing or inertgas.

f isfactory results being obtained with a rezine sulphidea y sulphideproduct (from the second stage)A heating 700 C.

Our present invention accordingly contemplates an improved method oflmaking zinc sulphide pigment in Whicha mixture temperature ofapproximately 'of zinc oxide pigment and sulphur is subjected to atwo-stage heat treatment operation in the first stage of Whichsubstantially.

complete conversion of Yzinc oxide to zinc sulphide is effected and inthe secondjstage of which an improvement in the color of the zincsulphide is effected. Each ,stage of the heat treatment operation isconducted under non-oxidizing conditions and preferably ata temperatureof G-800 In our present preferred practice of the invention, the twostages of the heat treatment operation are 'separate and independentlsteps, and thezinc sulphide product resulting from the first stage iscooled to atmospheric temperature before being subjected to thereheating or remuilling treatment. The raw zinc sulphide product(resulting from' the first stage) should be cooled below the oxidationtemperature of ,zinc sulphide (about 450 C.) under nonoxidizingconditions, so 'as to inhibit during cooling any appreciable oxidationof the Similarly the finished zinc should be cooled under conditionsinhibit- .ing any appreciable oxidation of the zinc sulphide therein.

The actual practice of the invention will. be better understood from thefollowing description representing what We now consider the preferredmanner of carrying out the invention.

Zinc oxide (preferably of a particle size of about 0.25 micron or lessand` carrying less than 0.1% of combined lead, cadmium f' and ironoxides or other impurities which would yield dark-colored sulphides) ismixed With aboutv three-quarters 'of its 1, Weight of finely dividedsulphur (preferably 4 tion losses.

should be carried `out in such a Way as toV y avoid too compact acharge. A satisfactory form of mixer is a rotary screen of the type usedfor boltin Zinc oxide, With a screen of about 14 mes es per lineal inch.Usually The reheating tempera. ture may vary from 600-800? C., very sattwo passes through the screen are advisable to insure a reasonablyuniform mixture.

vFailure to provide a sufficient excessv of sulphur or' to avoidcompacting of the charge during mixingofthe zinc oxide and sulphur mayresult in loW conversion of the zinc oxide to zinc sulphide. f

The mixed zinc oxide and sulphur is now charged into cylindricalretorts, of steel or other appropriate metal, or of refractory clay,carborundum, silica, or the like. Zinccoated or galvanized steel retorts4 inchesv in. internal diameter and` 4 feet in length have been usedwith satisfactory results. The retorts are closed at one end and arefitted with a removable cap at the other end, having a small opening topermit the 'zoo escape of the sulphur dioxide gas evolved in thereaction. r The retorts are completely filled With the mixed zinc oxideand sulphur, but the mixture is not packed under pressure in theretorts.

When the retorts have been filled, they are placed in a furnace andappropriately supported therein. This furnace is referably maintainedatfa'temperature o from 700 to 8000. The charge of mixed zinc oxide andsulphur should be heated rapidly to a temperature that will insurecomplete reaction before volatilization losses have reduced theamount ofavailable sulpliur belouAT that chemically equivalent to the zinc oxideto.- be converted. The temperature should be maintained at the reactionpoint only long enough to insure complete conversion of the zi c oxideto zine sulphide. Continuation of t e heating may result inrex-oxidation of the -zinc sulphide unless extreme precautions are takento exclude air. Excessive temperatures should be avoided as they resultin a light sintering together of the particles and the growth of largeparticles at the expense of small particles. Excessive temperatures alsoresult inthe early destructionof the apparatus and contamination of thezinc sulphide by the products of the reactions involved (formation ofscale, slags, etc.). i

Control of the vtemperature and -of the time of heating permits thecontrol of the particle size of the nal zinc sulphide pig'- A ment. Ashort time of heating and low temperature yields fine particle sizematerial, While a long time of 'heating and hightemperature causesthegrowth of particles and the sintering together of particles and hence arelatively coarse final product.

The retorts are allowedto remain in the furnace until the temperature atthe center ofthe charge has risen to the desired reaction temperature,say approximately 7 00 C.,

and the evolution of sulphur dioxide gas has ceased. The retorts aretheniwithdravvn fromthe furnace and the vent hole in the cap plugged toprevent free access of air,

, The caps are then removed and the resulting product, consisting forthemost'partof zinc sulphide, is discharged into suitable containers.This crude zinc-sulphide product is screened through a fine sieve toremove possible bits' lof scale and iron sulphide, and to mix itthoroughly for sampling. sample of the product is tested for residua-lzinc oxide and another sample is rubbed down in oil to determine thehiding power, and if the product is found on-grade in these respects,-it -is placed in stock for the reheating or' remuliiing operation. Forahigh grade product the residual zinc yoxide should not be more than 1%.The crude zinc sulphide product resulting from the first stageheat-treatment operation is now subjectedv to the second stageheat-treatment operation which isessentially a reheatmg Aor remuffiingoperati'on. This -reheating or remuflling of the crude. zinc sulphideproduct involves subjecting the product under non-oxidizing conditionsVto a temperature of from 60G-800 C., and cooling the thus reheatedproduct under conditions inhibiting any'oxidation ofA the zinc sulphidetherein. The operationA may be carried out in various ways, and we willnow describe certain-specific procedures employed by us in actualpractice.4

According to one procedure, the reheating vor remuffling of the crudeZincsulphide product is carried outfin an electrically heated furnacehaving acylindrical muifle' or heating chamber aboutve inches indiameter and four feet in length. The muflle is filled with the crude'zinc sulphide product, Vand appropriate connections are made .to pass aslow stream of superheated steaiu therethrough. The desired atmosphereof superheated steam within the muiiemay be obtained by passing a slowlstream of steam through a heated tube maintained at a red heat andthence to the muie. The muiil'e is 'carefully .sealed to exclude air andonly a small vent is provided for the escape of the superheated'steam.The temperature within the munie is gradually brought up to about 700C., at which time the heat is cut ofl' and the furnace is allowed tocool slowly, the current of steam, but 'of approximately lowertemperature, being maintained" through the munie. When the temperaturehas fallen to apoint where there is no danger of .oxidation uponexposure to air (about 40G-450 C.) the'mufile is opened and the reheatedproduct withdrawn. l

According to another procedure, we utilize a rotary furnace in which thecrude zine sulphide product is passed in a continuous stream throughatubular muffie, externally heated by electric heating coils or otherconvenient means. The'speed of rotationv and the angle4 of inclinationof the furnace axis 70 'are adjusted-to give a suitable rate of pas- Astufling boxwith a receiving hopperhaving a bottom discharge which issealed against the entrance of air by the layer of zinc sulphideaccumulated above the discharge damper. Steam or other non-oxidizing gasis introduced into the tubular Inutile, the flow being regulated to usethe minimum `amount that will prevent the entrance of air' .and thusavoid the oxidation. Ifsteam 1s used, provision is madeto maintain thecon-- 'tents of the ldischarge hopper at a temperature sufficiently highto avoid condensation of `the steam. This is accomplished either' bycovering the exterior of the hopper with a layer of insulation or bysurrounding it with heating devices, such' as electric resistance units,steam coils, steam jackets, or the like.vv l

In accordance with our preferred lprocedure', the crude zinc sulphideproduct is reheatedor rem-uffiedin an externally heated vertical munie.The crude sulphide is fed 10o by gravity lfrom a hopper placed above thetop'I of themuflle tube, and progresses down- ,wardly through the mui'leas fast as t e Afinished product is withdrawn from the bottom of themuflle.: 4To avoid danger of bridging and hanging of the charge iii the105 mufle tube, a mechanically operated'ra'pping device is installednearthe top of the 4muiile tube, by 'means of which the tube is struckat. frequent intervals, thus jarring it suiiic'iently to keep thechargefreely moving. o Below the heated zone'of the mufiletube, which is.vordinarily maintained at a temperature from 60G-800 C., the tube extendsdownward in air for several feet to provide a cooling Zone in which` thetemperature o thevcharge is reduced below the oxidation temperature ofzinc sulphide before discharging into the atmosphere. A mechanicallyoperated discharging device is installed at the-lower end of the muliiedtube 2 0 and is adjust-able to give any desired rate of passage of thematerial through the tube.

It has Vbeen found unnecessary to introduce any steam or. otherprotective atmosphere, o in the vertical mufie type of apparatus. f Themoisture `absorbed by the crude sull phide while in storage togetherwith the sulphide dioxide gas evolved by the reaction betweenvthesmallamounts of residual Zinc oxide and sulphide provide sufficientpro-- tection against oxidation. Furthermore, the fineness of the crudezinc sulphide product prevents any passage of air upward through thecharge eventhough the discharge end of the muiile tube is not sealedair-tight.

The vertical tube muiie may, if desired, be arranged to carry out thetwo stages of Vthe heat treatment operation f characteristic of thepresent invention. In carrying out the invention in this type ofapparatus, the mixture of zinc oxide pigmant and sulphur is charged atthe top of the muflie tube. The mixture passes downward into a heatedzone where the temperature is rapidly raised to a point at which thereaction between the zinc oxide and sulphur readily proceeds. Acontinuation of the mule tube above the point of introduction of thecharge serves as a stack to carry away the gaseous products of thereaction. If desired,.this stack may be connected by suitable lues to asulphuric acid plant for the recovery of the sulphur dioxide gas. Belowthe reaction zone of the mufl'le tube, the charge (now crude zincsulphide) passes into a second heated zone in which the temperature maybe controlled independently of the temperature in the re- -ac'tion zone.Here the second stage or remuiiiing operation takes place, and thefinished product proceeds downward through a cooling zone andappropriate discharge mechanism.

The invention may be carried out in val rious types and forms ofapparatus. In the accompanying drawings, we have illustrated certainforms of apparatus with which we have successfully practiced theinvention. In these drawings- Figs. 1 and`2 are side andv end sectionalelevations respectively of an apparatus for carrying out the first stageof the complete method of the invention.

4Fig. 3 is a side elevation, partly in section, of an electricallyheated rotary muile or retort for carrying out the second stage of themethod ofthe invention.

Fig. `l is a sectional elevation of la vertical muiile for carrying outthe reheating or remutfling stage of the invention, and

Fig. 5 is a sectional elevation of anapparatus of the vertical muiietype for progressively practicing the complete two-- stage heattreatment operation characteristic of the invention.

other end is provided with a removable cap 17. The cap 17 has a venthole 173| for the escape of the gaseous products of the reaction betweenthe zinc oxide and sulphur. The retortsv 17 are supported within thechamber 16 on tiles" or piers 18. The chamber 16 has doors 19 throughwhich the retorts are introduced intov and withdrawn from the furnace.The exhaust gases escape from the chamber 16 through a stack 20, and maybe disposed oflin any appropriate manner.

The reheating or remuiiiing apparatus of Fig. 3 comprises a rotatablymounted retort or muii'ie 25 surrounded by an electrical heating element26. The retort may conveniently be a zinc-coated steel tube about fourinches in internal diameter and aboutl six feet in length. of whichabout live feet is covered with appropriate heat-insulating material 27.The retort 25 with its heatinsulating cover 27 is provided with rings ortires 28 which are supported upon and adapted to be rotated by groovedrollers 29.

The rollers 29, on one side of the retort, are 1 secured to a shaft 30adapted to be driven from any suitable source of power, whereby theretort is appropriately rotated.

The crude zinc sulphide product is charged into the retort through ahopper 31 and screw conveyor 32. A cylindrical casing 33 encloses thescrew conveyor and extends through a stuiing box 34 on 4the end of theretort, whereby the entrance of air into the retort during the chargingof the' f crude sulphide is effectively avoided.

The ends of the electrical heating element 26 are connected to twoelectrically insulated slip rings 35 secured to the rotatable retort (orits heat insulating cover) nea-r the discharge end thereof. A air ofspring contacts 36 engage the slip rmgs 35 and serve to therebyelectrically connect the heating element 26 to an appropriate source ofelectric energy, diagrammatically indicated in Fig. 3 by the cables 37.

The discharge end ofthe retort 25 is connected to a stationary dischargepipe 38 through a stuiing box 39. The lower end of the pipe 38terminates in' the accumulated discharged sulphide in a covered pan 40,whereby the discharge end of the retort is etfectively sealed to avoidthe 'entrance of a1r. v

The retort is mounted at an appropriate inclination to insure thepassage of the zinc sulphide therethrough as the retort is rotated.Preferably the inclination of the retort is adjustable, so that desiredvaria. tions therein may be conveniently obtained.;

Superheated steam, or other appropriate' inert gas is introduced .intothe retort through the stationary pipe' 41, havingfl appropriateheat-insulating covering,- 42.'

Only suiicientgsteam is-admittedg tojinaintain within the retort anon-oxidizing atweighted closure 48 therefor, and ash-pit 49.

A Hue 50 conducts the gaseous products 4 l,of combustion from thefire-box to the retort or muiile chamber 51. An upright or verticallydisposed report or mufiie 52 is mounted within the chamber 5 1. Theretort 52 may conveniently lbe a zinc-coated steel pipe about fourinches in internal diameter and about ten feet long. -The gaseousproducts of combustion rom the burning fuel on the grate 46 pass throughthe tlue 50 into the chamber 51 and surround the retort 52 and escapefromthe top of the chamber 51 through an opening 53 to an appropriatestack (not shown);

The retort 52 is provided at its upper end with an extension 52, Thecrude zine sulphide product is charged into the lower end of theextension 52a through a hopper 54 andfscrew conveyor 55. The upperportion of the extension 521 serves as a stack for the escape of thegaseous products resulting from the reheating or remutliing operation.

vA rapping mechanism 56 'is .arranged to periodically strike theextension 52a for preventing the brid ing and hanging of the charge inthe mu e tube.

The lower lend lof the muie tube 52 communicates with an extension 52bexposed to the atmosphere below the furnace structure of themuftlefchamber. The extension 52b is provided with a mechanicallyactuated starwheel discharge 57. The extension 52b terh minates -at itslower end within a covered pan 58 where an effective seal is provided`by the accumulation of the discharged sulphide in the pan. The cover ofthe pan `58 is provided with 4a hinged portion 58', pery mittingconvenient access to the interior of the pan for leveling the materialtherein or -for removing material therefrom.

The vertical muiiie apparatus of Fig. 5. isgenerallysimilar in.construction to the apparatus of Fig. 4, and corresponding elementsareindicated in these two figures of the drawings bythe same referencecharacter. The apparatus of Fig. 5 is adapted to progressively carry outthe two stages of the heat treatment operation of the present invention,and the vertical retort 52 is accordingly longer than the retort 52 ofthe apparatus of Fig. 4. The retort 52 may conveniently be a zinc-coatedsteel tube -about -nine inches in diameter and about thirty feet long. l

The furnace structure of the heating chamber 51 isprovided with a Hue-60for conveying a portion of the heating gases portioning the relativeamounts of the fire gases admitted to the muffle chamber.

The zinc sulphide product resultingfrom the two-stage heat treatmentoperation of two points of the our present invention'may be subjected toany appropriate subsequent treatment. The subsequent treatment of thezinc sulphide product depends to some extent upon the use to which thezinc sulphide pigment is to be put. The product may, for example, befirst screened and then subjected to dry disintegration. Ordinarily, itis our preferred practice to wet 'grind the zinc sulphide product in apebble mill to break up aggregates. 'lhe ground productis then ltered,dried, and disintegrated. l

The zinc sulphide pigment obtained by the practice of our inventiontusing zinc oxide of an average particle size of 0.25 micron or less asa base) is a white powder having no visible crystalline character evenwhen examined under the highest magnification with the microscope. Itsultimate particles have an average diameter ofthe order lof 0.5 micron.It has strong pigment properties whenV rubbed down in oil, suchasineness, freedom from grit, and high hiding power or strength. Inwhiteness and brightness', it is. comparable with the best whitepigments available. The pig-- ment is essentially`pure zinc sulphidecontainingusually less Athan 1% of free Zinc oxide, and when made fromsubstantially purestarting materials contains only a few hundredths of1% of lead and iron.

1. The method of--rn'aking a pigment composed for the most part of zincsulphide which comprises heatin a mixture of zinc' oxide pigment andsulp ur to a suficiently high degree to effect the ready formation ofzinc sulphide, and subjecting the vresulting product to a heat treatmentoperation under non-oxidizing conditions.

2. The method of making a pigment composed for the most part of zincsulphide which comprises heating a mixture of zinc oxide pigment andsulphur to a sufficiently high temperature to effect the ready formationof zinc sulphide, subjecting the resulting product to a heat treatmentoperation under non-oxidizing conditions, vand cooling the heat-treatedproduct under conditions inhibiting any appreciable oxidation of theZinc sulphide therein.

3. The method of making a pigment coml which comprises subjecting amixture of lzinc oxide pigment and sulphur to a twostage heat treatmentoperation in the first stageof which substantially complete conversionof zine oxide to zinc sulphide is effected and in 'the second stagewof`which an improvement in the color of the zinc sulphide is attained, vandcooling the product' result-ing from the second stage under conditionsinhibiting any appreciable oxidation of the zinc sulphide therein.

4. The method of making a pigment composed for the most part of zincsulphide.

which comprises subjecting a mixture of zinc oxide pigment and sulphurto a twostage heat treatment operation under nonoxidizing conditionsand'at a temperature of GOO-800 C., substantially complete'conversion ofzinc oxide to zinc sulphide being effected in thejfirst stage of saidoperation and an improvement in the color ofthe zincv sulphide beingattained in the second stage,

and cooling the product resulting from the second stage under conditionsinhibiting any appreciable oxidation of the zinc sulphide therein.

5. The method of making a pigment composed for the most part of zincsulphide which comprises subjecting'a mixture of zinc oxide pigment ofan average particle p size not exceeding about 0.25 micron and -sulphurto a t/Wo-stage heat treatment operation in the first stage of whichsubstantially complete conversion of zinc -oxide to zinc sulphide iseffected and in the second stage of which an improvement 'in the colorof the zinc sulphide is attained, and cooling the product lresultingfromjthe second stage under conditions inhibiting any appreciableoxidation of the zinc sul hide therein. j

6. The method of making a pigment com.-

posed for the most part of lzinc sulphide? which comprises subjecting amixtureof yzinc oxide pigment .of any average particle size notexceeding about 0.25 micron and'` sulphur to a two-stage heat treatmentoperation under non-oxidizing conditions and at a temperature of 60G-800C., substantially complete conversion ofy zinc oxide to zinc sulphidebeing effected in the first stage of said operation and an improvementin the color of the zinc sulphide being attained in the second stage,and cooling the product resulting from the second stage under con-lditions inhibiting any. appreciable oxidation of the zinc sulphldetherein.

7 The method of making a pigment composed for the most part of zincsulphide Which comprises subjecting'a mixture of zinc oxide pigment ofan average particlesize not exceeding. aboutl0.25l micron and sulphur toa two-stage heat treatment operation in the first stage of Whichsubstantially complete conversion of zinc oxide to zinc sulphide iseffected and inthe second stage of which an improvement in the color"ofthe zinc sulphide is attained, cooling the product resulting from thesecond stage under conditions inhibiting any appreciable oxidation ofthe zinc sulphide therein, and disintegrating the cooled product andobtaining thereby a zinc sulphide pigment ofl zinc oxide to zincsulphide being eected,4

in the first stage of saidoperation and an` improvement in the color ofthe zinc s111- phide being attained in the second stage, cooling theproduct resulting from the second stage under conditions inhibiting anyappreciable oxidation of the' zincsulphide therein,

and disintegratinggthe cooledL product andlobtaining thereby a zincsulphide pigment of an average particle size not exceeding about 0.5micron. l

9.y The method of' making a pigment composed for the most part-of zincsulphide which comprisesl heating a mixture of zinc not 'exceedingabout.0'.25.micron and sulphur to a suiiciently hightemperature toeffect the ready formation of zinc sulp 1de,

`oxide pigment of an average particle size.

`and subjecting the resulting product toa f heatytreatment operationunder non-oxidizing conditions.4 j

10; The methodof making a pigment composedl for the mostrvp'art ofv zincsulphide which comprises heatmg a mixture of zinc oxide pigment ofyanaverage particle size not exceeding about.0.25 micron and finely dividedsulphur to a sufliciently high temperature to effect the ready formationof zinc sulphide, subjecting the resulting prod- -uct to a heattreatment operation under non-.y

oxidizing conditionsVand cooling the heattreated product underconditionsinhibiting any appreciable oxidation of the zinc `sulphidetherein.

which comprises heating a mixture o zinc -oxide pigment and sulphurunder non` oxidizing conditions toa. temperature of 600-800" C., coolingthe resulting product i under non-oxidizing conditions to atemperature-,less than 45000., then subjecting the under` non-oxidizingconditions at a. temperature of 60G-800 C., cooling the reheated productunder conditions-inhibiting any ap- 11. The method of making a pigmentcomposed for the most part ofA zinc sul hidel resulting product to areheating treatment f preciable oxidation of the zinc-sulphide 130,

v, therein, and distintegratingA the cooled product.

12. The method of making apigment composed for the Amost .part of zinesulphide `which comprises heating a mixture of zine 'l posed for themost part of zinc sul hide which comprises heatinga mixture o' zincoxide pigment of an average particle size not-exceeding about 0.25micron and sulphur under non-oxidizing` conditions to-a temperature of600-800 b., cooling the resulting product under conditions inhibitingany appreciable oxidation of the zinc sulphidel therein, then`subjecting the resulting product -to -a reheating treatment under lnonoxidizing conditions at a temperature of 60G-800 C., and cooling thereheated product Aunder conditions'inhibitin'g any appreciable oxidationof the. zinc sulphide therein.

14. The method of making a pigment composed for the most part of zincsul hide which comprises heating a mixture o zinc oxide pigment of anaverage particle size notexceeding about 0.25 micron and finely dividedsulphur under non-oxidizing conditions to a temperature of GOO-800C.,vcool ing the resulting product underconditions Iinhibiting anyappreciable oxidation of the under non-oxidizing conditions at atemperal v therein,

zinc'sulphide therein, then subjecting the resulting product to areheating treatment ltureJ of G-800 0.,' cooling the reheated productunder conditions inhibiting any a preciable .oxidation of thezincisulphie and disintegratingthe cooled product v and pigment of anaverage particle size not exceeding about 0.5 micron.

15. The improvement in the method of making a pigment composed for themost art of.zinc sulphide which comprises sub- ]ecting `a zinc sulphideproduct "resulting from-the interaction of zinc oxide pigment andsulphur to aheat treatment operation conditions, and 1 obtaining therebya zinc sulphideunder non-oxidizing conditions and at a temperature ofGOO-800 C., and cooling the resulting product under conditionsinhibiting any appreciable oxidation of the `zinc sulphi e therein.

16. The improvement-in vthepjmethod of making a pigment composed for themost part of zinc sulphide which comprises progressively passing a zincsulphide product the interaction of zinc oxide resulting from pigmentand sulphurthro'gh an upright'reaction chamber and subjectingthe product-whilepassing through the chamber to a sufciently high temperature toeffect an improvement In the color thereof. v

17. The improvement in 'the method Aof making a pigment composed vforthe most part of zinc sulphide which comprises subjecting a. zincsulphide product resulting .from the interaction of zinc oxide pigmentand sulphur to a heat treatment-operation under,non-oxidizingconditionsand at a sufliciently ;high temperature to eiect'4 an improvement in thecolor of the product, and

cooling the thus heat-treated productl under conditions inhibitinganyappreciable oxidation of lthe' zinc sulphide therein.

making a pigment composed for the most part of zinc sulphide whichcomprises` rogressivelypassing a @inc sulphide pro uct from theinteraction of zinc oxide pigment and sulphur through any uprightchamber heated to a temperature of 600- 800 C., andv in which anon-oxidizing atmosphere is maintained.

18. The improvement in the method of 19. The improvement in the methodof.

making a pigment composed for the most part of zinc sulphide whichcomprises progressively passing by gravity a zinc sulphide productresulting from the interaction of zinc oxide pigment and sulphur throughan upright chamber heated to a temperature of 60G-800 C., and in which anon-oxidizing atmosphere is maintained, progressively discharging theresulting product from the bottom of the chamber.,

any appreciable oxidation phide therein.

FRAK GJBREYER. s CLAYTQN w. FARBER.

and cooling they re- -sulting product under conditions inhibiting ofthezinc -sulr v. 1 los In testimony whereof we aliix our signatures. f f

